Recovery of organic acids with an amine solvent



mama Sept. 13, 1951 UNITED STATES PATENT, OFFICE i zmas 4 a Event F. Smith, Chicago, and Vanderveer Vowhees, Homewood, IILaIIgnors-to standardoil Company, Chicago, 11]., a connotation of 1-- Application April 20, 1049, Serial No. 80,288 cum. (cl. M)

This invention relates to the recovery of organic acids. More particularly, it relates to the separation of organic acids from water-immiscible mixtures thereof with non-acidic organic substances.

Mixtures of organic acids with various other organic materials are widely encountered in the various branches of the chemical process industries, and their separation is in many cases a difilcult problem. For example, crude petroleum and variousvpetroleum fractions contain small proportions of organic acids, which must be removed in order to avoid corrosion diiiiculties in the storage and utilization thereof. The air-oxidation of petroleum fractions by various processes yields liquid products that are rich in organic acids and aldehydes. In the hydrogenation of carbon monoxide according to the Fischer- Tropsch process and its various modifications, particularly when the hydrogenation is'carried out in the presence of an alkali-promoted iron catalyst, the resulting liquid hydrocarbon product ordinarily contains a substantial proportion of organic acids, the recovery of which is complicated by the concurrent presence of a wide variety of other organic oxygenated compounds. The separation of acids from any of these mixtures is conventionally carried out by extraction with an aqueous solution of an alkaline material, such as sodium or potassium hydroxide; and thereafter the acids are liberated from the aqueous extract by treatment with a strong acid, such as sulfuric acid. This process is open to the economic objection that it wastes a mole of inorganic base and a mole of inorganic acid for each mole of organic acid recovered. Moreover, the use of such materials leads to severe corrosion problems in actual practice. To avoid these difliculties, it has heretofore been proposed to effect the extraction of organic acids from organic mixtures with an aqueous solution of an organic base, suitably a water-soluble amine, and thereafter to recover the organic acids from the aqueous extract by steam distillation, by treatment with carbon dioxide, or the like. This improvement, however, is also uneconomical in the use of large quantities of steam; and the recovery of the amine and the carbon dioxide from the resulting amine carbonate solution leads to additional process diiiieulties.

We have now discovered that normally waic'rimmiscible organic acids may be stratified from an aqueous amine solution thereof by heat treatment, optionally with water dilution. We have further discovered that such normally water-immiscible organic acids are effective solvents at elevated temperatures for extracting the water-soluble lower organic acids from aqueous amine solutions. On the basis of these discoveries, we have devised a'new process for the recovery of organic acids from organic solutions thereof which may include the following steps:

1. Extraction of the charging stock with aqueous solution of a water-miscible amine.

2.Back-washingoftheresulfingextractwitha liquid hydrocarbon to ranove therefrom any organic oxygenated compolmds other than organic 3. Cquntercurrent extraction of the resulting purified aqueous extract at elevated temperature with a water-immiscible organic acid, with or without water dilution, whereby the greater portion of the acids are liberated from the amine and areselectively utracted by the water-immiscible organic acid,

4. Fractionation of the resulting organic extract to separate the lower-boiling organic acids, which are withdrawn, and a higher-boiling organic acidfractiomwhichmaybeusedinpart as the "water-immiscible organic acid" required for step 3.

5. Stripping of residual organic acids from'the depleted amine solution obtained in step 3, and recycling of the regenerated amine solution.

One object of our invention is to recover or ganic acids from water-immiscible mixtures thereof with other organic compounds. Another object is to separate organic acids in purified condition from immiscible mixtum thereof with other organic compounds. A further object is to separate organic acids from other types of organic ongenated compounds. A still further object is to reduce the consumption of chemicals in the recovery of organic acids. A subsidiary object is to separate organic acids into fractions of higher and lower molecular weight. An additional object is to isolate individual organic acids in substantially purified conditions, free from other organic acids, other organic compounds, and other impurities. Other objects will be apparent from the description and the claims.

Suitable amines for use in the extractant solution of step 1 of our process, as defined above, are thebroad class of water-immiscible, substantially non-volatile amines, including (hydroxy alkyl) amines, such as monoethanolamine, diethanolamine, triethanolamine, 2-amino-1-propanol, 3-amino 1 propanol, l-amino 2 propanol, 2- amino-2-methyl-1-propanol, 2-amino-2-methyl- 1,3-propanediol, and the like, and the alkanepolyamides, oxazolines,

amines, such as ethylenediamine, 1,2-propanediamine, N-substituted derivatives thereof, and the like; 01' this broad class, we prefer to use secondary or tertiary amines, since they have less of a tendency to undergo side reactions during the course ofsubsequent operations at elevated temperatures, with the consequent production of and other undesired by-products. For all of these reasons, triethanolamine is the preferred active constituent of our extractant solution.

The concentration of amine in the extractant solution may range between wide limits, suitably between around'2 and 40 percent by weight or higher, but is preferably between about 5 and 25 percent; and a percent solution has the advantage of extracting the organic acids in an efficient manner without simultaneously extracting objectionable quantities of other organic oxygenated compounds. Below about 5 percent by weight, emulsion difficulties are encountered, the extraction efllciency of the solution is impaired, and an excessive quantity of extractant solution is required for a given extraction. Above about 25 percent by weight, the extractant solube greatly reduced or entirely avoided by step 2 of our process, as outlined above, wherein the extract is back-washed with a liquid hydrocarbon, preferably at a temperature within the range disclosedabove as suitable for step 1, and

' optimally at the same temperature as that emtion tends to become excessively viscous, and r the subsequent separationof dissolved acids from theextract becomes increasingly difficult. Extractant solutions having higher proportions of the water-miscible amine are operative, nevertheless, in the extraction operation of step 1; and when employed, the resulting extract may thereafter conveniently be diluted with water to a suitable concentration, either prior to or during the heat-treating step, in order to facilitate the recovery of organic acids therefrom.

The extraction operation of step 1 is preferably carried out at ordinary or somewhat reduced temperatures, in order to diminish the tendency toward emulsion formation, and in order to eifect the maximum extraction of acids per unit volume of extractant solution. It will be apparent, however, that the extraction temperature must besumciently high to avoid any substantialgelling of the liquidphases. It will be further apparent hereinafter that the extraction efliciency is greatly impaired at substantially elevated temperatures, for example above about 60 C., owing to the fact that the organic acids are readily released from the extract at elevated temperatures above about 75 C. For these reasons, we ordinarily choose to operate at a temthat such liberated acids stratify and dissolve perature within the range of about 10 to 60 Q,

and preferably between about 20 and C.

Ordinary pressures may satisfactorily be employed in step 1. -Where, for any reason, ebullition may tend to occur within the extraction zone, elevated pressure should preferably be employed, suflicient to repress such ebullition, in order to prevent disturbance of the liquid phases therein, which would tend ordinarily to destroy the extraction emciency.

The volume of extractant solution employed should contain a quantity of amine at least sumcient to react stoichiometrically with all of the organicacids in the charging stock. We prefer to employ a substantial excess of the amine, for example up to around twice the theoretical quantity, in order to insure substantially complete removal of the organic acids from the charging stock.

Where our charging stock contains organic. oxygenated compounds other than organic acids. the resulting aqueous extract tends to be contaminated therewith. Buch contamination may ployed in stepl. Since the organic acids are chemically bound to the amine in the extract, whereas the contaminants are not. the contaminants are readily removed by the hydrocarbon, leaving the extract in purified condition. The back-washing operation may be carried out with substantially any hydrocarbon that is liquid under the process conditions. A suitable hydrocarbon fraction may be isolated, for example. from the charging stock itself. where the charging stock is of the hydrocarbon type. We prefer to use a comparatively narrow hydrocarbon fraction, boiling at least about 10 C. away from the boiling point of the major constituent of the acid fraction. Such a hydrocarbon fraction, when dissolved in small quantities by the aqueous extract and carried therewith into subsequent processing steps, may readily be separated from the desired organic acids by fractional distillation.

In step 3 of our process, the aqueous amineplus-acid extract from step 2 is countercurrently contacted at an elevated temperature with an organic acid that is insoluble therein under the temperature conditions employed. For this purpose, a normally water-immiscible organic acid is suitable, or a higher-boiling organic acid fraction, obtained for example in a later step of our process by fractional distillation of the recovered organic acids. In this connection, the term water-immiscible" is to be understood as meaning not totally miscible with water in all proportions) The temperature employed in step 3 should ordinarily be sufliciently high to induce liberation of the normally water-immiscible organic acids from their amine salts, so

in the countercurrent extractant solution. At the same time, the countercurrent extractant solution removes from the aqueous extract the lower-boiling organic acids contained therein which do not tend to stratify under the conditions employed. In this way, before the aqueous amine solution leavesthe extraction zone it is contacted last with a higher-boiling organic acid relatively insoluble therein, so that the emerging amine solution is comparatively free from organic acids, and may be reutilized in step 1. The

acid stream leaving the countercurrent extraction zone comprises, under properly chosen conditions of temperature and relative space velocities, substantially all of the organic acids entering the zone.

The temperature required to produce liberation and stratification of the organic acids from. the aqueous extract in step 3 depends largely upon the amine employed, the concentration thereof in the aqueous extract, and the types of organic acids present therein. The temperature varies directly as a function of the amine concentration and inversely as a function of the molecular weight of the acids. organic acids ranging from about C1 to C20 and having an average molecular weight of 145 was found to stratify from an aqueous 20 percent by weight triethanolamine solution at C., whereas it did not stratify from an aqueous 50 percent by weight triethanolamine solution even at 98 C. In general, stratification may be induced at tem- For example, a mixture of" lower-boiling organic acids in the totalacid mixture is lower.

It will be apparent that step 3, as set forth above. is a specialized adaptation of the effects on which our invention is based, namely, the heat stratiflcation of normally water-immiscible organic acids from aqueous amine solutions, and

the extraction of lower organic acids from aqueous amine solutions-by use of normally water-immiscible organic acids at elevated temperatures. Manifestly, we may utilize, these effects in a simpler manner to accomplish the objects of our invention. we may, for example, heat the purified extract from step 2 to an elevated temperature so asto liberate and stratify higher organic acids therefrom. Subsequently, after withdrawal of the stratified acids, the depleted extract may be further extracted with a higher organic acid to remove lower organic acids therefrom. In the last operation, the temperature employed need only-be sufliciently high to prevent the solution of substantial quantities of the higher organic acid in the aqueous amine solution. Other modifications will be apparent to those skilled in the art.

In step 4, the acid mixture from step 3 is fractionally distilled by conventional methods to separate therefrom a, higher acid fraction for re-' cycling.to step 3. Simultaneously, a lower'acid fraction is obtained. It will be apparent that this fractionation may be carried out in such a way as to separate the constituent acids as sub-.

stantially pure fractions.

In step 5, the depleted amine solution obtained in step 3 is stripped of residual acids by steam distillation, or by blowing with a hot, inert gas, or by similar methods, and the purified aqueous amine solution is recycled to step 1. Alternatively, the greater portion of the depleted amine solution obtained in step 3 may be recycled to step 1 while the remainder thereof is withdrawn and exhaustively stripped of acids before being recycled, in order to prevent the buildup of substantial quantities of acid therein.' In the latter case, the exhaustively purified amine solution should be recycled to such a point in the extraction zone of step 1 that the charging stock is contacted with it last before emerging from the extraction zone.

Our process is suitable in general for the separation and recovery of organic acids from organic mixtures thereof, including the broad classes of carboxylic acids and phenols. We may, for example, employ our process for the purification of hydrocarbons, alcohols, ethers, and the like; by separating therefrom phenol, cresols, and the like, and/or alkanecarboiwlic acids, such as acetic, propionic, butyric, isobutyric, valeric. caproic, caprylic, capric, myristic, palmitic, and. stearic acids, and the like, and their olefinic and hydroxylated homologues, and/or alkanepolycarboxylic acids, such as oxalic, malic, maleic, succinic, adipic, and sebacic acids, and the like, and/or other carboxylic acids, such as benzoic acid, cyclohexanecarboxylic acid, and the like.

The attached flowsheet illustrates an advantageous embodiment of our invention:

,A charging-stock mixture of "hydrocarbons.

aseaoos I O aldehydes, alcohols, ketones, carboxylic acids, and phenols, such as the organic phase produced by the hydrogenation of carbon monoxide in the presence of a fluidized, alkali-promoted iron catalyst, is introduced through line llll into a lower intermediate section of extractor I02, wherein it flows upward countercurrent'to an aqueous extractant solution containing 10 percent by weight of triethanolamine, introduced into an upper intermediate section of the extractor through line III. Extractor lllis maintained at a temperature around 20 to 30. C. and at autogenous pressure.

A quantity of water is introduced into the top of extractor I02 through line IIM to wash out any entrained or dissolved amine from the aciddepleted charging stock, which then emerges from the top of the extractor through line I05 and is withdrawn, suitably ijor additional processing to recover other organic oxygenated compounds therefrom.

Through line I08 into the bottom of extractor I" is introduced a hydrocarbon fraction to back .w'ash from the extract stream any organic The back-wash stream flows upward through extractor III, and after being enriched with organic oxygenated compounds from the downward-flowing aqueous extract, it blends with the charging-stock hydrocarbons and emerges overhead through line l ll5. i

The aqueous extract stream emerging from the bottom of extractor Hi2 through line Ill comprises predominantly water and triethanolamine organic-acid salts. This stream is transferred by pump I08 through line I09 and heater HI into the top of extractor I l I, where it is countercurrently contacted at an elevated temperature with a stream of higher molecular weight organic acids, obtained as described below, and introduced into the bottom of extractor Ill through line H2. The temperature within extractor III is suitably maintained between about and 200' (1., and preferably between about and C. At these temperatures, the amine salts are highly unstable, and virtually all of the organic acids, other than the members of lowest molecular weight, such as acetic acid, tend to form a separate phase. This phase, and the loosely bound and dissolved lower molecular weight acids, are extracted by the higher molecular weight acid stream as it flows upward through extractor ill. The resulting mixture of organic acids flows out of the top of the extractor through line 3, and is led through heater H4 into an intermediate section of fractionator H5, equipped with reboiled IIB. Therein, the lower molecular weight organic acids are distilled overhead through condenser lll into reflux drum 8, from which a portion is refluxed to the top of fractionator I I5 through valved line I I9, and the remainder is withdrawn through valved line In to storage or further processing to isolate the individual constituents thereof. A higher molecular weight organic-acid fraction, containing propionic acid and higher homologues, flows m, by which it is through line :24,

heater I28, and line H! into the bottom of extractor III. whereit serves as a countercurrent extractant, as described above. The remainder of. the stream in line III flows through cooler I28 and is withdrawn through valved line I21 to storage or'to further processing to isolatethe individual constituents.

The acid-depleted emerging from the bottom of extractor III through line I20 is transferred by pump I20 through line I30 and heater ISI into an intermediate section of stripper I32. Within the stripper, any organic acids remaining in the aqueous stream are distilled out by the action of reboiler I, together with a suflicient quantity of water to adjust the concentration of the amine to the desired level for recycling. The mixture of acids and water emerge overhead through condenser I34 into separator I25, where two layers, are formed. The acid layer is withdrawn through valved line I36 and 'commingled with the acids in line III. The aqueous phase is refluxed in part to the top of stripper I32 through valved line I31, and is withdrawn through valved line I38.

A substantially acid-free aqueous amine solution emerges from the bottom of stripper I32 through cooler I39 and line I40, and is recycled by pump III through line I03 to an upper intermediate section of extractor I02. Makeup amine is supplied as needed through line I42 to line H0.

While we have described our invention with reference to a particular charging stock, a particular amine extractant solution, and a particular flow scheme, it is to be distinctly understood that we are not limited thereto. Our invention is broadly applicable to the problem of recovering organic acids, as set forth indetail herein, and is not to be construed more narrowly than the disclosure of the entire specification, including the appended claims. In general, it may be said that any-modifications or equivalents that would ordinarily occur to one skilled in the art are to aqueous amine solution ganic extract and separating therefrom said the amine concentration therein to not more than 25 percent by weight. heatingsaid extract to an elevated temperature above about '16 C.

whereat liberation and stratification of said organic acid takes place, and separating and withdrawing said organic acid.

9. In a process for separately recovering a water-miscible organic acid and a water-immiscible organic acid from a water-immiscible solution comprised thereof, the steps which comprise (1) extracting said organic acids from said solution at a temperature between about 10 and 60 C. with an aqueous extractantsolution containing between about 5 and 25 percent by weight of. a a water-miscible amine; (2) countercurrently contacting the resulting aqueous extract at an elevated temperature above about 75 C. with a water-immiscible organic acid, and withdrawing an organic extract containing substantially all of the organic acids initially present in said aqueous extract; (3) fractionally distilling said orwater-miscible organic acid and said waterimmiscible organic acid; and (4) recycling a portion of said water-immiscible organic acid to said step, 2 for countercurrently contacting said aqueous extract.

10. In a process for recovering organic acids 354mm a hydrocarbon solution thereof obtained by be considered as lying within the scope of our invention.

In accordance with the foregoing description, we claim as our invention:

1. In a process for separating a water-immiscible organic acid from a water-immiscible organic solution comprised thereof. the steps which comprise extracting said organic acid from said solution at a temperature between about 10 and 60 C. with an aqueous extractant solution containing between about 5 and 25 percent by weight of a water-immiscible amine, heating the resulting extract to an elevated temperature.

above about 75 C. whereat liberation and stratiflcation of said organic acid takes place, and separating and withdrawing said organic acid.

6. The process of claim 1 wherein said water miscible amine is an alkanediamine.

'l. The process of claim 8 wherein said alkanediamine is ethylenediamine.

8. In a process for separating a water-immis- I cible organic acid from a water-immiscible organic solution comprised thereof, the steps which hydrogenating carbon monoxide inthe presence of an alkali-promoted iron catalyst, said hydrocarbon solution containing water-miscible and water-immiscible organic acids, the steps which comprise (l) countercurrently contacting said hydrocarbon solution'at a temperature between about 10 and 60 C. with an aqueous extractant solution containing between about 5 and 25 percent by weight of a water-immiscible amine, whereby an aqueous extract is obtained containing substantially all of the organic acids initially present in said hydrocarbon solution; (2) backwashing said aqueous extract with a liquid hydrocarbon, whereby non-acidic impurities are removed from said aqueous extract; (3) countercurrently contacting the resulting aqueous extract at an elevated temperature above about 75 C. with a water-immiscible organic-acid fraction and withdrawing an organic extract containing substantially all of the organic acids initially present in said aqueous extract; (4) fractionally distilling said organic extract and separating therefrom a water-miscible organic-acid fraction and a water-immiscible organic-acid fraction; and (5) recycling a portion of said water-immiscible organic-acid fraction to said step 3 for countercurrently contacting said aqueous extract.

VANDERVEER V00.

REFERENCES crrsn The following references are of record in the file of this patent: 

1. IN A PROCESS FOR SEPARATING A WATER-IMMISCIBLE ORGANIC ACID FROM A WATER-IMMISCIBLE ORGANIC SOLUTION COMPRISED THEREOF, THE STEPS WHICH COMPRISE EXTRACTING SAID ORGANIC ACID FROM SAID SOLUTION AT A TEMPERATURE BETWEEN ABOUT 10 AND 60* C. WITH AN AQUEOUS EXTRACTANT SOLUTION CONTAINING BETWEEN ABOUT 5 AND 25 PERCENT BY WEIGHT OF A WATER-IMMISCIBLE AMINE, HEATING THE RESULTING EXTRACT TO AN ELEVATED TEMPERATURE ABOVE ABOUT 75* C. WHEREAT LIBERATION AND STRATIFICATION OF SAID ORGANIC ACID TAKES PLACE, AND SEPARATING AND WITHDRAWING SAID ORGANIC ACID. 